1. Field of the Invention
The present invention generally relates to the field of gas turbine engines for the generation of electricity and, more particularly, to a method for controlling engine rotor or turbine drive shaft acceleration in an offload condition.
2. Description of Related Art
A turboalternator may be configured as an engine by placing a load on a drive shaft driven by a gas turbine engine. When configured in this manner, the turboalternator supplies mechanical energy to an external device, as well as continues to create electrical energy. Several different control systems are often utilized in starting a gas turbine engine. A typical startup procedure of a gas turbine engine includes the steps of: (1) rotating the engine rotor or turbine drive shaft through the use of an external source until enough compressed air is supplied to the combustion chamber for combustion; (2) combining the energy supplied from the external source with energy created through combustion until the gas turbine engine reaches self-sustaining speed; and (3) further accelerating the gas turbine engine using only energy created through combustion until synchronous speed is reached.
When a turboalternator is configured as an engine, an operator of a turboalternator will typically remove or reduce the load from the drive shaft without shutting down the turboalternator. It is generally desired that the turboalternator continue to operate at a constant speed.
When the load is removed or reduced, the turbine drive shaft will immediately accelerate because of the continuous energy supplied to the turbine drive shaft from combustion within the combustion chamber of the gas turbine engine. A prior art method of controlling the acceleration of a gas turbine engine when a turboalternator is configured to be an engine includes the step of setting the fuel valve to a position that allows very little fuel into the combustion chamber. Controlling the offload acceleration in this manner often results in the loss of combustion in the combustion chamber, typically referred to as blowout, or overheating in the combustion chamber. When blowout occurs, the gas turbine engine must be restarted.
It is, therefore, an object of the present invention to avoid blowouts in the combustion chamber of a gas turbine engine during an offload condition by controlling the acceleration of the turbine drive shaft.
A method for controlling the speed of a drive shaft of a gas turbine engine during an offload condition utilizes a turbine compressor, an annular combustor, and a control system containing a PID (Proportional Integral Derivative) controller. The annular combustor can include a single fuel source or multiple fuel sources.
Several steps are required prior to operating the gas turbine engine. First, a table of minimum fuel flow rates that will not cause blowout is created. The table""s parameters include: (1) rotational speed; and (2) inlet temperature. A function is then created to determine the fuel valve position based on a fuel flow rate accessed from the table. Finally, a variance from synchronous speed must be defined and placed into the control system.
Once all of the necessary variables are defined, a load is placed on the drive shaft of the gas turbine engine and the drive shaft is accelerated to synchronous speed. A sensor can be used to determine when the load is removed. A control system for controlling the speed of the drive shaft during an offload condition is then enabled.
The control system controls the speed of the drive shaft during an offload condition by the following steps: (1) determining if the current speed is within the predefined variance of synchronous speed; (2) exiting the control system if the current speed is within the predefined variance; (3) enabling a PID controller within the control system; (4) requesting a speed for the drive shaft; (5) requesting a fuel valve position based on the requested speed through the PID controller; (6) requesting a fuel flow rate from the table; (7) requesting a fuel valve position through the function; (8) positioning the fuel valve according to the fuel valve position requested through the PID controller if the requested position does not allow less fuel to the combustion chamber than the fuel valve position requested through the function; (9) positioning the fuel valve according to the fuel valve position requested through the function if the fuel valve position requested through the PID allows less fuel through to the combustion chamber than the fuel valve position requested by the function; (10) turning on an ignitor if the fuel valve position used was requested through the function; (11) maintaining the burning of the ignitor until the fuel valve position used is requested through the PID controller; (12) turning off the ignitor if the fuel valve position used is requested through the PID controller; (13) maintaining the ignitor in an off position until the fuel valve position used is requested through the function; and (14) repeating steps 1 through 13 until synchronous speed is reached.